Particle sensors are for example used to measure the level of pollution in the air.
Low-cost systems are available based on the measurement of light scattered at the particles which pass along with an air flow through a detection volume in the sensor. This air flow is for example driven by a fan or a heater. Each particle is illuminated by a continuous light source and creates a light pulse with a duration determined by the passage time of the particle through the detection volume.
These pulses are amplified, filtered and counted in an electronics system. In addition to indicating the particle concentration level, such detectors can be used to drive the operation of an air purifier device.
Optical particle sensors typically give a particle count as the sensor output, and these numbers are then converted to mass concentration. However, this conversion usually deviates from reality, as different types of aerosol have different densities. This is a generic problem for all optical particle sensors.
Moreover, low cost optical sensors, are typically only sensitive for particles above a certain particle size (for example 700 nm). The so-called PM2.5 standard however needs to provide the mass of all particles below a size of 2.5 μm. In practice, particles below 700 nm do contribute to this mass, however they are not detected by the sensor. Therefore, the conversion of the number of particles counted by the detector to the total particle mass will be affected by the relative contribution of these small particles to the overall PM2.5 level. This contribution depends on the distribution of particle sizes in the air, which in turn depends on the type of pollution event.